Vibratory conveying devices are known in the art. U.S. Pat. No. 4,313,535, which is incorporated by reference herein, teaches a typical vibratory conveyor of the excited frame type. Vibratory conveyors are generally used for conveying particulate product in a commercial environment. Generally speaking, vibratory conveyors consist of a conveying member, or bed, for supporting the product as the product is conveyed along a given path of travel. The conveyor bed is generally in the form of a unitary elongated channel, or trough, for containing or directing the product. In the food processing industry, the unitary construction of the vibratory conveyor bed is considered advantageous because it is viewed as more sanitary than conveyors which utilize endless belts, rollers, or other multi-piece conveying members.
In the conventional design of an excited frame vibratory conveyor, the conveyor bed is supported by and attached to the upper ends of a series of parallel leaf springs. The lower ends of the leaf springs, in turn, are attached to a frame which rests on a floor or other supporting surface. The leaf springs extend upward from the frame and are generally inclined toward the intake end of the conveyor bed, which is generally horizontal and parallel to the floor.
Attached to the frame in a conventional excited frame vibratory conveyor is a drive assembly which produces a vibrating force which imparts a reciprocal motion to the conveyor bed. The vibrating force is usually produced by the rotation of a pair of counter-rotating eccentric weights, which are sub-components of the drive assembly. Each of the eccentric weights has an axis of rotation and a center of mass which is eccentric to the axis of rotation. The distance from the axis of rotation to the center of mass is generally referred to as the eccentricity.
As each weight is rotated about its axis, the eccentric configuration of the center of mass causes a centrifugal force to be produced relative to the axis. The magnitude of the centrifugal force is directly proportional to both the mass of the rotating weight and the distance from the axis of rotation to the center of mass of the weight. The magnitude of the centrifugal force is also proportional to the square of the angular velocity of the weight as it is rotated about the axis. Thus, the equation for the centrifugal force can be written as follows: F.sub.c =mr(.omega..sup.2), where "F.sub.c " is the centrifugal force, "m" is the mass of the weight, "r" is the distance from the axis to the center of mass of the weight, and ".omega." is the angular velocity of the weight about the axis. For weights with fixed masses and fixed centers of mass, the terms "m" and "r" combine to form a constant. Within the industry, this constant has been referred to as the "eccentricity times the weight," or the "ERWT" value. With this in mind, the equation for the centrifugal force can also be written as follows: F.sub.c =ERWT(.omega..sup.2). Thus, the centrifugal force produced by a rotating eccentric weight is proportional to the ERWT value of the weight and to the square of its angular velocity.
If two eccentric weights of substantially similar ERWT values are rotated about parallel axes in opposing directions and are properly synchronized, the centrifugal forces produced by each weight will combine to form a substantially linear sinusoidal vibrating force. To illustrate, consider two such weights in a standard three-coordinate axis system. First, it is assumed that both axes are parallel to the x-axis and are in the x-y plane and are supported by a common support. Next, a reference line on each weight is defined as being perpendicular to the respective axis and extending from the axis to the center of mass of the respective eccentric weight. Also, it is assumed that the weights rotate in opposite directions and at the same angular velocity. Furthermore, as the weights rotate, the reference line on each weight is parallel to, and in the positive region of, the z-axis at the same point in time. Thus, as the weights rotate, they produce centrifugal forces in the y-z plane.
However, because of the rotational synchronization of the weights, the forces parallel to the y-axis will be canceled out with respect to the common support of the weights, since any force produced in the positive y-direction by one weight is counteracted by an equal force produced by the other weight in the negative y-direction. Conversely, the centrifugal forces in the z-direction are additive, since both weights produce a force in the positive z-direction at the same time, and vice versa for the negative z-direction. Thus, the centrifugal forces of the weights will combine to produce a linear, sinusoidal, vibrating force in the z-direction.
The equation for this sinusoidal force produced by a pair of synchronized, counter-rotating eccentric weights can be written as follows: F.sub.s =[F.sub.c1 sin(.alpha.)]+[F.sub.c2 sin(.alpha.)], where "F.sub.s " is the sinusoidal force, "F.sub.c1 " is the centrifugal force produced by the first weight, is "F.sub.c2 " is the centrifugal force produced by the second weight, and ".alpha." is the angle of the weight from a position where both centrifugal forces oppose each other and are parallel to the y-axis. If the ERWT values of both weights are the same, then F.sub.c1 =F.sub.c2, and the equation can be written: F.sub.s =2F.sub.c sin(.alpha.), where "F.sub.c " is the centrifugal force produced by each weight. Substituting the equation above for F.sub.c, the equation becomes: F.sub.s =2mr(.omega..sup.2)sin(.alpha.), or F.sub.2 =2(ERWT)(.omega..sup.2)sin(.alpha.), where the ERWT value for each weight is the same.
In conventional excited frame vibratory conveyors, as discussed above, the direction of the vibrating force produced by the drive assembly is generally oriented such that it is directed along a line which passes through the center of mass of the conveyor bed, and the underlying frame. The resulting motion of the conveyor bed is then generally upward and toward the exhaust end of the conveyor in one direction, and downward and toward the intake end of the conveyor in the opposite direction. This reciprocal motion of the conveyor bed tends to "bounce" the particulate product along the conveyor bed from the intake end to the exhaust end.
While excited frame vibratory conveyors of conventional design have been operated with varying degrees of success, there have been shortcomings which have detracted from their usefulness. For example, if delicate products, or products with coatings, are conveyed on a conventional excited frame vibratory conveyor, the delicate products can be damaged, and coatings can separate from the products intended to be coated. This problem is due, in large part, to the repeated impact or bouncing of the product against the conveyor bed as the product travels to the distal end thereof.
Therefore it has long been known that it would be desirable to provide a vibratory conveyor device which achieves the benefits to be derived from similar prior art devices, but which avoids the detriments individually associated therefrom.